METHOD FOR DETERMINING THE OPTICAL INPUT POWER OF AN APD RECEIVER

Abstract: The invention relates to a method for determining the optical input power of a receiver provided with an avalanche photo diode (11). To make an accurate and simple determination of the optical input power possible, (a) an equation between the optical power and the current (Iapd) of the avalanche photo diode is determined for the receiver circuit used, which equation is processed in such a way that a reference magnitude representing setting an operating point of the diode and a magnitude representing the breakdown voltage (Vbr) of the diode constitute a ratio therein, (b) a first calibration step is performed by determining at a high input level a value for responsivity (r) occurring in said equation, which value gives the value corresponding to the input power received by the diode, (c) a second calibration step is performed by determining said ratio at a low input power level, and (d) subsequently, the optical input power is determined by determining the current (Iapd) of the avalanche photo diode and by calculating from said equation the optical power corresponding thereto, by using the calibrated values.

Description: Method for determining the optical input power of an APD receiver The invention relates to a method for determin- ing the optical input power of a receiver provided with an avalanche photo diode. An Avalanche Photo Diode (APD), being a light sensitive semiconductor diode, is generally used e.g. as a detector in receivers of long-haul optical line equipments. The most usual types of avalanche photo diodes are Germanium Avalanche Photo Diode (Ge-APD) and Indium Gallium Arsenide Avalanche Photo Diode (InGaAs- APD). In comparison to e.g. a PIN photo diode, an ava¬ lanche photo diode provides an improved sensitivity, which is due to a multiplication of photoelectric cur¬ rent occurring in the avalanche photo diode. For the use of a line equipment, it is of bene¬ fit to know the optical power received by the device. The average power received by an APD can be calculated from the current of the diode and from the voltage across the diode, when the characteristic curve of the diode is known. However, a problem is that the multiplication of the photoelectric current increases very abrubtly when the voltage across the diode approches the break¬ down voltage of the diode. Even a small error in the measurement of the voltage thus leads to a great error in the calculation of the optical power. The resolution re- quired by the measurement and the magnitude (from some twenty or thirty volts to over a hundred volts) of the voltage (absolute value) makes an accurate measure¬ ment particularly difficult. Since the value of the breakdown voltage changes as a function of temperature, even an accurate measurement of the voltage would give an incorrect result at different temperatures. For providing an accurate measurement result, a method has been developed (European Patent Application 0 360 877), for instance, in which a low-frequency cur¬ rent signal is superimposed on the photoelectric current and the received optical power is calculated from the resulting voltage signal. A drawback of such a solution is, however, that the circuitry required for a practical implementation is very complicated. The input power can also be determined accord- ing to a precompiled table, by searching in the table for the optical power corresponding to a measured oper¬ ating point of the diode. Because the proportion of the multiplication as well as responsivity (responsivity represents the ratio of photoelectric current to optical power, if no multiplication takes place, in other words, how high an output current is received from a diode having a predetermined optical input power, when multi¬ plication is not taken into consideration) vary as per receiver, the tabulation method is inaccurate when con- ventional calibration arrangements are used. Additional¬ ly, separate tables have to be worked out for avalanche photo diodes of different types, which makes an imple¬ mentation into practice difficult. The object of the present invention is to get rid of the drawbacks described above and to provide a method by means of which it is possible to determine accurately the optical input power of an avalanche photo diode receiver without tabulation or complicated circuit arrangements. This object is achieved by the method according to the invention, which is characterized in what is set forth in the characterizing portion of claim 1. The idea of the invention is to determine at first, on the basis of known equations, an equation be- tween the current and optical power of a receiver cir- cuit used, Which requires that (a) a voltage acting over a diode can be solved as a function of current and (b) that this function contains a certain reference magni¬ tude, which represents setting an operating point for the APD. Then the equation is processed in such a way that said reference magnitude and a magnitude represent¬ ing the breakdown voltage of the diode constitute a ratio therein. In a carefully designed receiver, this ratio is constant regardless of temperature, and there- fore, temperature needs not be considered at the calcu¬ lation of power (in an example described below, this is realized by means of a temperature-compensated bias voltage source, the output voltage of which tracks changes in the breakdown voltage). Consequently, it is not necessary to know the value of the breakdown volt¬ age, but the correct constant value of said ratio is searched for at the calibration stage. In this way, the difficulty is avoided that separate manufacturers of avalanche photo diodes inform the value of the breakdown voltage typically for a certain current, while according to theory, the current is infinite when the diode is supplied with a voltage the magnitude of which corre¬ sponds to the breakdown voltage. Subsequently, the re¬ sponsivity is at first calibrated from the processed equation and then said ratio. The optical input level can then be determined within the whole range of input power and operating temperature by measuring the current of the avalanche photo diode at first. In avalanche photo diode receivers, a biasing of a diode is typically regulated in such a way that the multiplication is strong, i.e. at its optimum as far as signal-to-noise-ratio is concerned, when the optical in¬ put level is low (at the sensitivity level) . (Biasing signifies setting an operating point for an avalanche photo diode. ) At high input levels, multiplication is restricted to be as low as possible for avoiding over¬ loading of a receiving amplifier. However, the multipli¬ cation factor of an indium gallium arsenide avalanche photo diode particularly must not be restricted to be too low (minimum multiplication factor is typically about 3, whereby the receiver still operates). In a preferred embodiment of the invention, biasing occurs by means of a series resistance, whereby a decreasing multiplication factor with increasing input level can be utilized for an accurate calibration of responsivity. (Multiplication weakens the relative accuracy of the calibration of responsivity, as appears later on). De¬ creasing multiplication with increasing input level also provides the advantage that the dynamic range required for the measurement of the APD current is reduced con¬ siderably, without the accuracy suffering anyway (opti¬ cal power is expressed in dBms, and therefore, it is not detrimental that the power measurement resolution de¬ creases with increasing input level. ) The invention and its preferred embodiments are described in more detail in the following by way of ex¬ ample with reference to the attached Figure 1 illustrat¬ ing a receiver circuit to be used in the method of the invention. In a circuit according to Figure 1, a receiver is biased by means of a temperature-compensated (a vari¬ ation in breakdown voltage value of an APD is compen¬ sated for) voltage source 13 via a series resistance R B . The anode of the avalanche photo diode 11 is connected to the input of a receiver amplifier 12 and the positive pole of the voltage source is connected via the series resistance R s to the cathode of the avalanche photo diode. The common pole of the series resistance and the avalanche photo diode is connected via a capacitor C to ground. The purpose of the capacitor is to prevent a signal to be received from entering the series resis¬ tance R s . A voltage drop of the series resistance de¬ creases the multiplication factor of the APD when the optical input level increases. In the avalanche photo diode ( 11 ) , photoelec¬ tric current is amplified by multiplication according to formulas ( 1) and (2) : I apd = M x I p ( 1 ) M = V 1 - a.pd 1 (2) V b, r where •J- apd = current of avalanche photo diode (in milli- amperes) M = multiplication factor of avalanche photo diode, depending on voltage across diode I p = non-multiplied photoelectric current (equal to that of corresponding PIN diode) V aPd = voltage across avalanche photo diode V br - breakdown voltage of avalanche photo diode η = constant, depending on semiconductor material used in avalanche photo diode. Input level (in decibels compared to milliwatt) is obtained when the effect of the multiplication factor M and the responsivity r is reduced from the current of the avalance photo diode: P ( dBm) = 10 x lg • apd * (ι_ (_ v = 10 x lg S __ a___£p.d?) ■ i) I X M V b, i (3) However, formula (3) is not usable as such, due to the above problems associated with the measurement of the voltage V apd of the avalanche photo diode, but the equation has to be solved as a function of the current I apd . In the circuit of the example, the voltage of the avalanche photo diode depends further on the voltage V b i as of the bias voltage source 13 and on the series resistance R s : ^apd ~ Vbias -s X ^apd ' (4) In equation (4) , the DC level of the amplifier input is assumed to be zero volts. However, a small de¬ viation needs not be taken into consideration, if the deviation remains unchanged or a possible change caused by temperature has been considered at the temperature compensation of the bias voltage. By combining the formulas (3) and (4) , it is obtained: P ( dBm) = 10 x lg -apd 1 - Vbias ∑apd X - η ( 5 ) v b,∑ In equation (5), a ratio is formed from the voltages V biaε and V br , which ratio can be calibrated without the value of the V br being known. The calculation of equation ( 6) presupposes a measurement of the voltage V bias , no very high accuracy is required, however, and it is enough that this mea¬ surement is performed in connection with calibration only. The current I apd can be calculated e.g. from a voltage measured over the series resistance R B . In the method according to the invention, the procedure is as follows. To begin with, the sensitivity of the receiver is tuned to the optimum value (i.e. the multiplication factor is tuned to its optimum value) by regulating the voltage V biaε of the bias source 13 to its optimum value. This voltage is then not changed any more, but this reg¬ ulated value is changed only if the temperature changes. Then the responsivity is calibrated, which means that, for the responsivity r occurring in formula (6), a numerical value is searched for, by which the result calculated by means of formula (6) corresponds to the actual optical power (transmitted to the ava¬ lanche photo diode) . (To make it possible to determine the value of responsivity from formula (6) , the diode current I apd and the bias voltage V blas are to be deter¬ mined and, for the parameter η, a value corresponding to the type of diode used is to be searched for. ) This calibration step is performed at a high input level, which makes the influence of multiplication slight (mul¬ tiplication factor M is as low as possible). In equation ( 6) , the ratio V biaE /V br affecting the multiplication can be provided for the time of calibration with a typical value of about 0,97...0,98 (calibration error caused by the inaccuracy of the value is slight in practice). (Op- timum sensitivity is found in the vicinity of the break¬ down voltage of the diode in the avalanche photo diode receiver. In practice, the ratio V biaE /V br is between 0,95 and 0,99, if the initial regulation of the bias voltage has been performed correctly. ) The bias regulation of the receiver with re¬ spect to the breakdown voltage of the avalanche photo diode, i.e. the ratio V biaε /V br , has an abrupt effect on the multiplication. To make it possible to determine the optical power correctly also at low input levels, the accurate numerical value of the ratio has to be known. After the calibration of responsivity, the ratio must therefore be calibrated at a low input level, where mul¬ tiplication is strongest. The calibration is performed by searching for a numerical value of ratio (V bias /V br ), by which value the optical power calculated according to formula (6) corresponds to the actual power (trans¬ mitted to the avalanche photo diode) as accurately as possible. To make it possible to determine the value of the ratio (V biaε /V br ) from formula (6), the current I apd of the diode has to be redetermined at this input level. At this calibration stage, the value obtained above for the responsivity is also utilized, for which reason the calibration steps shall be performed in the order de- scribed above. Accordingly, though it is not known on the basis of formula (6) in principle, which is the influ¬ ence of the responsivity r on the final result, on the one hand, and which is the influence of the multiplica- tion factor on the final result, on the other hand, formula ( 6) and the biasing manner used (in which the voltage across the diode is regulated according to the power level) offer an opportunity for a simple calibra¬ tion, because the value of the responsivity can be searched for in a situation when the influence of the multiplication factor is known to be insignificant. Because the input level is solved based on the physical behaviour of the multiplication, these calibra- tions are sufficient for solving the current/light char¬ acteristics of the avalanche photo diode within the whole input level range. At low input levels, a restric¬ tion is an error caused by a leakage current of the diode, which is, however, of practical significance only for a germanium avalanche photo diode at a high tempera¬ ture. After the calibrations, the input level can then be determined by measuring the current I apd of the diode and by calculating the input power on the basis of formula (6) by using the measured current value. The current can be measured in many ways (known per se), e.g. as described above by measuring the voltage accross the series resistance R s and by determining the current on the basis of the measured voltage. (At determination of input level, an accuracy of the order of 1 dBm can be achieved in practice. This presupposes that the volt¬ age of the bias source is appropriately temperature- compensated. ) An additional advantage of the solution of the invention is that it applies to different kinds of ava¬ lanche photo diodes regardless of breakdown voltages and optimum biasings, if only the parameter η relating to the semiconductor material is correct. The accuracy of this parameter is not critical, however, and its value can be inquired of the manufacturer of the avalanche photo diodes (e.g. from data sheets of the manufactur¬ er). Typical values for germanium avalanche photo diodes are between 2,5 and 8 and for indium gallium arsenide avalanche photo diodes between 1 and 3. Though the invention has been described above referring to the embodiments of the attached drawings, it is obvious that the invention is not restricted to them, but it can be varied within the scope of the in- ventive idea set forth above and in the attached claims. In principle, the solution according to the invention can be applied correspondingly also to other biasing manners comprising regulating means for regulating the bias voltage according to the optical input level. De- pending on the situation, the above-mentioned ratio can also have a very different expression, according to which magnitude represents the setting of the operating point of the diode and, in turn, which magnitude repre¬ sents the breakdown voltage of the diode. In the biasing according to the above example, the direction of the bias voltage and the direction of the diode can also be opposite.

Claims: Claims : 1. A method for determining the optical input power of a receiver provided with an avalanche photo diode (11), c h a r a c t e r i z e d in that an equation between the optical power and the current ( I apd ) of the avalanche photo diode is determined for the receiver circuit used, which equation is pro¬ cessed in such a way that a reference magnitude repre- senting setting an operating point of the diode and a magnitude representing the breakdown voltage (V br ) of the diode constitute a ratio therein, a first calibration step is performed by deter¬ mining at a high input power level a value for a respon- sivity (r) occurring in said equation, which value gives the value corresponding to the input power received by the diode, a second calibration step is performed by de¬ termining said ratio at a low input power level, and subsequently, the optical input power is de¬ termined by determining the current ( I apd ) of the ava¬ lanche photo diode and by calculating from said equation the optical power corresponding to said current, by using the calibrated values. 2. A method according to claim 1, c h a r ¬ a c t e r i z e d in that the bias voltage (V blaB ) is supplied via a series resistance (R ε ) to the avalanche photo diode (11), whereby a value is determined for the ratio V biaε /V br of said bias voltage and the breakdown voltage of the avalanche photo diode at the second cali¬ bration step.

Inventors: KIVIJAERVI ANTTI, KIVIJAERVI, ANTTI
Applicants: KIVIJAERVI ANTTI, NOKIA TELECOMMUNICATIONS OY, KIVIJAERVI, ANTTI
Application year: 1994
Main classification: H04B10
IPC codes: H04B10